Here's a basic explanation of the cycle:
Your fish produce waste, which is ammonia. Ammonia is lethal for fish, but a beneficial bacteria grows on your tank's surfaces (mainly the filter media) that converts the ammonia into nitrites, Unfortunately, nitrites are also lethal. Another good bacteria converts those to nitrates. A low level of nitrates in a tank is acceptable (under 20). Some live plants will process nitrates.
Ultimately, you want your water readings to be 0 ammonia, 0 nitrites, and less than 20 nitrates. If you have any ammonia or nitrites in your tank (which you do), your tank is not cycled. That means that there isn't a big enough bacterial colony to eat all the harmful waste byproducts yet. Algae is common in an uncycled tank, as is pH fluctuation.
Your goal at this point is to get your tank cycled. 10 gallons a week is definitely NOT too much for a weekly
water change in a 55 gallon - in fact, most will say it isn't enough.
I would say the first thing to do is to stop listening to the pet store people. Almost every person on this website has gotten rotten advice from them. I think most have their hearts in the right place, and just don't know better. Some just want to make more money, and the more goofy products they can sell you, the more money they make. And a very few actually know what they are doing and will give you sound advice. Unfortunately, the advice you are getting isn't good.
Cycling a tank with fish in it is harmful to the fish. The exposure to ammonia and nitrites burns their gills, and can kill them or leave them with permanent organ damage and shortened lives. The old-school method of cycling was to just leave the fish in the tank, hope they survived, and let the ammonia levels rise high to cycle the tank quickly.
This method is considered by many to be cruel, as it clearly harms the fish. When you are not able to do your cycle fishless (which is what is recommended), because you already have fish living in your tank, a more humane method of cycling is to do daily partial water changes. If you do this, you remove some of the harmful toxin from the water, making it safer for your fish. It extends the time it takes to cycle, but is much less traumatic for the fish in your tank.
If you do the partial water changes with the addition of ammolock or Prime every day, that's even better because then you leave the ammonia in the water for the bacteria to feed on, but you detoxify it so it hurts the fish less.
Since the good bacteria live on the filter media, you never want to change this out completely. Just cut out any carbon and continue to use the cartidge. Many people only replace a cartidge once every few years. All you need to do is rinse the cartidge in discarded (dechlorinated) tank water every few months when it gets gunky. If you change the old cartidge and throw it away, you are throwing away all the good bacteria every few weeks.
edit: I'm not familiar with this disease. I hope this isn't what did your poor fish in. I'm sorry if it did.

I think the information I gave you will still help your tank, though, even if it was DGD.
Lucy, leave it to you to come in here with a simple explanation while I type a novel

